Each lot of OPV is tested for neurovirulence by intraspinal inoculation of monkeys. We have investigated the possibility of replacing monkeys with an in vitro assay based on the differential susceptibility of g-IFN-treated human neuronal cells to infection with strains of type-3 poliovirus with different levels of virulence. The study also established persistent infections of neuronal cells that may be useful in studies of novel mutations that appear during such infections.